This invention relates to explosive circuit opening devices, sometimes called disconnectors, for use with excess voltage protection devices such as surge arresters or lightning arresters.
Surge arresters have limits on their fault current conduction capability which if exceeded can result in a large pressure buildup in the porcelain arrester housing. The housing may shatter and, in some locations, endanger personnel and equipment.
An electrical disconnector is a known type of device used in association with surge arresters. The disconnector includes an explosive element that explodes on an excess fault current being conducted between the arrester and the ground terminal at the end of the disconnector. One form of known electrical disconnector is that described in Carothers et al U.S. Pat. No. 3,679,938, July 25, 1972, which includes an explosive cartridge, that is a metal shell containing an explosive material such as gun powder. Upon an excess current the cartridge explodes and ruptures the disconnector housing to remove the ground terminal from the arrester. Such a disconnector terminates the fault current but may not do so without having the arrester housing shatter as well.
The present invention relates to a new disconnector device particularly for distribution class lightning arresters which are relatively small size, high volume products that must be economically manufacturable. A drawback of the known prior art that uses explosive charges is that they are not self-clearing devices but instead they require the operation of some other device on the source side of the system such as a fuse cutout, recloser, or breaker to actually clear the fault that results in the operation of the disconnector. This is the case because the gunpowder of the explosive cartridge creates a highly conductive path of ionized gas. The ionized gas may continue the fault current conduction for a period even after operation of the disconnector with an arc stretching up to about one meter or more. This phenomenon can still result in the explosive fracture of the arrester porcelain housing which is highly objectionable. A period of continued fault current conduction of about 5 to 12 cycles can occur depending on where the failed arrester is located in the system. The fault current could be 10,000 amperes or more. Thus the object of avoiding shattering of arresters has not been attained by the prior art except in instances in which stronger, shatter resistant arrester housings are used at a substantial cost increase.
The present invention is directed to a more positive solution to the shattering porcelain problem. A disconnector in accordance with the invention comprises a cylinder containing compressed gas as the key element to the success of this new design and in place of the explosive cartridge of the prior art disconnectors. The gas cylinder is in the conductive path from the arrester components to the ground terminal. In one embodiment it has a seal that defines the breaking point of the gas cylinder which upon a certain excess fault current will release CO.sub.2, or other dielectric gas, into the disconnector cup. The pressure increases in the cup causing it to fracture and physically force the ground cable away from the arrester. Just prior to this, however, the buildup of dielectric gas pressure inside the disconnector cup serves another more important function in forcing the arc of the fault current conduction, that is the arc through the spark gap in the disconnector, to be extinguished. This means there can be much greater assurance of protection of the porcelain arrester housing against shattering. It also means that the functions required by protective devices elsewhere on the system such as breakers, reclosers or fuse cutouts can be reduced.